Low temperature anomaly of LO phonons in La1.85Sr0.15CuO4 and YBa2Cu3O7

Inelastic neutron scattering measurements of La1.85Sr0.15CuO4 and YBa2Cu3O7 show that the dispersion of the high energy LO phonon mode along the (1,0,0) direction is strongly temperature dependent, and at low temperatures develops an anomalous feature indicative of dynamic cell-doubling. The anomaly does not change through superconducting transition and gradually disappears between 50 and 250 K in LSCO. Possible implications are discussed.